Precision metal casting molds

ABSTRACT

A process for producing ceramic shell molds for use in the casting of metals which involves the steps of building up a ceramic shell mold by freezing a plurality of superimposed stuccoed layers onto an expandable pattern and removing the pattern by the application of heat.

United States Patent lnventor Alan K. Cutler Calumet City, 111.

Appl. No. 837,298

Filed June 27, 1969 Patented June 8, l97l Assignee Nalco ChemicalCompany Chicago, Ill.

PRECISION METAL CASTING MOLDS 6 Claims, No Drawings U.S. Cl 164/26 Int.Cl B22c 9/04, B22c 9/12 Field of Search 164/8, 23, 24, 25, 26

[56] References Cited UNITED STATES PATENTS 2,912,729 11/1959 Webb164/25 X 3,512,571 5/1970 Phelps 164/8 X Primary Examiner-J. SpencerOverholser Assistant Examiner-John E. Roethel Attorneys-Johnston, Root,OKeeffe, Keil, Thompson and Shurtleff, John G. Premo, Charles W. Connorsand Morando Berrettini PRECISION METAL CASTING MOLDS INTRODUCTION Theprecision casting of metals may be defined as a process whereby a moltenmetal is poured into a mold, wherein it solidifies to produce a castmetal object which is characterized as having close specifications as totolerance, detail and sur face quality. One of the most common methodsof precision casting is precision investment casting, which is sometimesreferred to as the lost wax process.

A specific form of investment casting, which embodies thc most modernform of the process, is the shell mold investment casting process. Thisprocess utilizes an expendable pattern which is a replica of the articleto be produced. The pattern is coated with a ceramic slip which is madeup of a granular refractory in a liquid binder vehicle. The coatedpattern is stuccoed with granular refractory material and the stuccoedcoat is allowed to dry. The process of coating and stuccoing is repeatedto build up successive layers which form a ceramic shell around theexpendable pattern. The ceramic shell thus formed is treated to removethe expendable pattern, leaving a thin mold capable of having many typesof molten metals poured directly into its cavity to produce precisioncastings of high quality. One embodiment of the shell mold investmentcasting process is disclosed and explained in detail in U.S. Pat. No.2,948,032.

The combination of the use of slip coating and stuccoing a patternresults in extremely good reproduction of the pattern face by the mold.It can be readily seen that this process of coating a pattern is notlimited to use in conjunction with expendable patterns or to the lostwax process. Precision molds can also be made wherein a shell mold asdescribed above is reinforced by investment in a backup material such asgranular refractory, insulating material or a castable. Another methodwould be to produce mold sections by coating and stuccoing patterns andusing the mold sections independently. More preferably, the sectionscould be used in conjunction with a backup material such as sand,plaster, a granular refractory or a suitable castable.

It is essential to the production of a high quality precision metalcasting that the mold surface be dimensionally correct. The mold surfacemust also be smooth, impermeable to the metal, and of such a chemicalnature that it does not react with the metal. For these purposes,various refractory materials may be used depending upon the chemicalnature of the metal which is being poured.

The problems of control of mold dimensions and mold surface, as well asthe problems of providing a mold which is chemically unreactive with themetal which is being cast, have been dealt with by the prior art. Themechanics of proper grain sizing, control of viscosity of slips andother techniques in the physical making of a mold have been developed toa high degree by the prior art and are not essential to the subjectinvention.

Although the shell mold investment casting process has been developed toa high degree by the prior art and is extremely useful in the making ofprecision metal castings, the process does have some shortcomings.Perhaps, the greatest shortcoming is involved in the time and effortnecessary to build up a composite shell mold by coating, stuccoing anddry i3efore successive layers can be added to a pattern to build up ashell mold by the prior art method of investment casting mold making,each earlier layer must be dried thoroughly. The drying of successivelayers involves the expenditure ofa good deal of time in the making of amold. For instance, a five-layer shell mold, wherein 1 hour is allowedfor each layer to dry before applying the next coating and stucco layer,would take a minimum of 5 hours for its manufacture, This 5-hour spaninvolves not only the use of a period of time but also the use of thequantity of space which is necessary to store the molds at a controlledtemperature and humidity, so that they may be dried properly.

OBJECTS It is an object of the subject invention to provide an improvedprocess for producing a ceramic shell mold for use in the investmentcasting of metals whereby stuccoed layers can be added to each other toform the finished shell mold without waiting for each preceding layer todry.

Another object of the subject invention is to provide a process forproducing a ceramic shell mold whereby a succession of coated andstuccoed layers are built up upon an expendable pattern with each layerbeing frozen prior to the ap plication of each successive layer.

Other objects will appear hereinafter.

THE lNVENTlON In accordance with the objects of the subject invention,an improved process for producing a ceramic shell mold for use ininvestment casting of metals has been discovered. The process of thesubject invention is one whereby a ceramic shell mold which is comprisedof a composite of successive stuccoed layers is made in a small fractionof the time which is necessary for the making of a similar ceramic shellmold by prior art processes.

The process of the subject invention involves forming a ceramic shell bythe steps of:

l. Coating an expendable pattern with a ceramic slip which comprises anaqueous colloidal silica sol or an aqueous alumina coated colloidalsilica sol vehicle having a granular refractory material suspendedtherein,

2. Stuccoing the coated pattern with a granular refractory material,

3. Freezing'the stuccoed coat at a temperature below about 4. Forming aplurality of superimposed similar coatings on the pattern, and

5. Removing the pattern'from the ceramic shell.

Aqueous colloidal silica sols of the type used as binders in thepractice of the subject invention are available commercially fromseveral sources and have been described in detail in the prior art. Athorough description of these colloidal silica sols is set forth in theaforementioned U.S. Pat. No. 2,948,032.

The sols are most usually prepared using the techniques in accordancewith the teachings of Bird, U.S. Pat. No. 2,244,325. Bird teaches thataqueous colloidal silica sols of high purity may be prepared by passinga dilute solution of an aqueous alkali metal silicate solution incontact with a cationic exchange resin in the hydrogen form.

The sols produced by the Bird patent are relatively dilute, but may beconcentrated using evaporation techniques such as, for example, thosedescribed in Bechtold, et al., U.S. Pat. No. 2,574,902 and Alexander,U.S. Pat. No. 2,601,235.

The particular method of preparation of the colloidal silica sols whichare useful in the subject invention is not essential to the invention.Since preparation of these sols is well known to the prior art and hasbeen described in detail in the patents cited, as well as in numerousother publications, the specific preparation procedures will not befurther discussed here.

The colloidal silica sol binders which are most useful in the practiceof the invention contain relatively large amounts of silica, so that theSiO content is between about l2 percent and about 60 percent by weight,with excellent results being obtained with silica sol binders containingbetween about 20 percent and about 60 percent by weight ofsilica.

The alumina coated sols which are useful in the practice of the subjectinvention as alternate binders to the colloidal silica sols are definedas those in which all of the silica particles are coated with at least acontinuous mololayer of alumina. The sols are further characterized ascomprising dense spherical particles of an average particle size of froml0-30 millimicrons and an alumina-to-silica ratio of from 0.10 to 0.50.More preferably, the sols are of a particle size of from l5-20millimicrons and have an alumina-to-silica ratio of from 0.10 to 0.30.An ideal sol, as shown below in Table I, has an average particle size of20 millimicrons and an alumina-to-silica ratio of0.20.

Table l ALl'MlNA COATED SOL Solids 20'7:

Al /SiO, Ratio 0.20

Viscosity at 77 F. c.p.s. Specific Gravity at 68 F. 1.!4

Specific Surface Area. l5(l m.*/gni. Average Particle Size '10millimicron Particle Charge Positive Density of Sol at 68 F. 9.5 lbs/galFreezing Point 32" F Na,O Content Less than 0.0 l i Chloride Content 0.3

A method of producing the alumina coated sols has been described byMindick, et al., in U.S. Pat. No. 3,139,406. However, it must beunderstood that the method of making alumina coated sols is notessential to the subject invention. lt is only necessary that thefinished sol be one in which all of the silica particles are coveredwith at least a continuous monolayer ofalumina.

The preferred alumina coated sols for use in the subject invention havea total solids content of from 5 percent-4O per cent by weight. Morepreferably, the sols should have a solids content of from percent-35percent by weight and most preferably. about percent by weight. The solsshould further have apH range offrom O.56.5 and more preferably, a pHrange offrom about 5.0 to about 6.0.

The alumina coated sols which are most useful in the subject inventionand their manner of use in the prior art method of manufacturing ceramicshell molds for use in investment casting is described in detail incopending application U.S. Ser. No. 619,143, now Pat. No. 3,445,250,filed Feb. 28, I967. The improvement in this application over thecopcnding application lies in the use of the freezing step and not inthe use ofthe binder or refractory.

The refractories which are useful both in the ceramic slip of thesubject invention and as a stucco material which is used in the processofthe subject invention are ofa variety of granular refractorymaterials. The particular refractory material to be used is chosen onthe basis of economic availability, ease of handling, chemicalreactivity with the metal to be poured and other considerations whichare not essential to the subject invcntion.

As has been pointed out above, it is not the particular colloidal silicasol nor is it the particular refractory material which is used which isessential to the subject invention. The novelty of the subject inventionlies in the freezing of each layer and the immediate application ofadditional layers to the previously frozen layers to provide a fastmethod of making a ceramic shell. Aside from the freezing steps whichare replacements for the prior art drying steps in the process ofmanufacturing a ceramic shell mold, the ceramic slips and granularrefractory materials which are used in the subject invention areessentially the same as those taught by the prior art.

The only variation which is provided by the subject invention is thatrefractory materials which would not be used in the prior art processbecause of their tendency to react with a colloidal silica or aluminacoated sol to form an unstable slip, might be used in the practice ofthesubject invention, because a slip might not have to be held in a stablecondition for as long a period of time. For instance, a magnesiumsilicate stucco material which might tend to gel a prior art coatingslip and thus prevent it from drying properly might be used in thepractice of the subject invention, because the fast freezing step wouldnot allow the magnesium silicate sufficient time to have a deleteriouseffect upon the slip.

Typical granular refractory materials which are useful in the subjectinvention are those selected from the group consisting of fused silica,crystalline silica, Zircon, mullite, alumina, calcinedalumino-silicates, magnesia, chromia and magnesium silicate. For use inthe ceramic slips of the subject invention, the grain size of thegranular refractory should, most desirably, not exceed 100 microns norshould it be less than 0.1 micron.

The amount of aqueous colloidal silica sol or alumina coated sol used inrelationship to the granular refractory may be varied over a relativelywide range. Usually, from about 20 percent to about 50 percent by weightofthe aqueous silica sol material may be used and from about 50 percentto about percent by weight of the granular refractory material. Theweight ratio of sol vehicle to ceramic material usually should bemaintained within the range of about I to l to about I to 3. It must beunderstood, however, that the weight ratio may vary depending upon aparticular granular refractory which is used, as, obviously, some of therefractory materials named above are considerably more dense thanothers.

The ceramic slips which are useful in the subject invention areessentially the same as the ceramic slips which are used in the priorart method of ceramic shell manufacture, such as that described in U.S.Pat. No. 2,948,032 and such as are wellknown to the prior art. Theparticular slip which is used is identical with that used in the priorart process. The novelty of the process of the subject invention lies inthe freezing of the stuccoed slip and not in any variation of the slipitself.

The granular refractory material which is used in stuccoing the coatedexpendable pattern and in building up successive stuccoed layers shouldhave a particle size within the range from about 50 to about 2000microns and should be of a material which is compatible with thegranular refractory material which is used in any particular slip whichis being stuccoed.

Other ingredients, which are known to the prior art, such as compatiblewetting agents, may be added to the ceramic slip to achieve betterwetting of patterns and the resultant better surface ofthe finishedshell mold.

Other materials, such as dyes, can be added to color the molds or toserve as indicators in the treatment of the molds. The addition of theseauxiliary materials does not change the character of the slip or thestucco material in its ability to be frozen to build up the shell moldby the process of the subject invention. Therefore, these materials neednot be described further.

After a sufficient number of refractory coats have been built up uponthe expendable pattern material and have been frozen, the expendablepattern must be removed from the resultant ceramic shell. In thepractice of the subject invention the expendable pattern is removed byheating the shell mold and the pattern to a temperature ranging fromabout 700 F. to about 2000 F. Where suitable equipment is available, itis preferred to conduct the pattern removal operation at a temperatureranging from about 1400 F. to about 1700" F. This removal of the patternby the application of heat can be performed in a suitable furnace or insome other apparatus such as an infrared heating oven. Furnaces whichcan be used are of types which are typically used in foundriespracticing the prior art method of shell mold investment casting and arewellknown to the art. The particular apparatus which is used is notessential to the subject invention.

The invention will be better understood with reference to the followingexamples.

EXAMPLE I A typical shell mold is made by the process ofthe subjectinvention as follows: 2500 milliliters of an aqueous colloidal silicasol having 30 percent SiO by weight is placed in a 6 liter steel beaker.13.5 pounds of a granular fused silica having an SiO content of 97.3percent by weight and a thermal coefficient of expansion of about 5X10,cm./cm./C., ground so that the largest particle present is no greaterthan 75 microns, is added using good mechanical stirring.

After the addition of the fused silica powder to the colloidal silicasol, the total volume of the slip is about 5 liters.

A clean wax pattern is dipped into the prepared slip and stuccoed with afused silica grain having the same characteristics as the fused silicaused in making the slip and having a particle size such that thesmallest particle size of the granular fused silica is no smaller than95 microns. The wax pattern, so treated, is immersed into a freezingchamber at 30 F. The coated pattern is held in the freezing chamber fora period of 10 minutes, removed, recoated with the slip and stucco andreturned to the freezing chamber. The process is repeated until a totalof five coats are deposited upon the wax pattern to form a frozenceramic shell mold.

The coated pattern is placed in a melt-out oven, the temperature ofwhich is approximately 1900 F. The wax is melted out of the shell moldwithin a period of a few minutes. The mold is held in the oven attemperature for approximately l5 minutes to decarburize the mold andremove any remaining residue from the wax. The mold is then removed fromthe oven and is ready to be used for the casting ofmctal.

EXAMPLE ll Another example ofthe preparation ofa ceramic shell mold bythe process ofthe subject invention is as follows:

A slip is made up using 2 /2 gallons of an alumina coated silica solhaving the physical characteristics given in Table l above and 50 poundsof-325 mesh tabular alumina.

The viscosity of the slip is adjusted by adding tabular alumina until anumber five Zahn cup viscosity of 10 seconds is obtained. The slip isallowed to stand overnight with continuous agitation in order to reducethe number of air bubbles contained therein.

Wax patterns are dipped in the slip and stuccoed with +70 mesh tabularalumina. The stuccoed coatings are placed into a freezing chamber at atemperature of -lO F. and held for a period sufficient to completelyfreeze the coating, approxi mately 7 minutes. An additional stuccoedcoat is added and frozen in place in the same manner and three backupcoats are applied in the same manner, except that :1 +100 mesh calcinedalumina silicate grog is used as a stucco material. The frozen stuccoedpatterns are placed into a burnout oven where the wax patterns areremoved completely from the ceramic shell mold. The shell mold is nowready to receive the molten metal to form a casting.

EXAMPLE Ill Other ceramic shell molds, for use with various metals andutilizing various colloidal silica sols and alumina coated sols asbinders and various granular refractory materials as replacements forthe fused silica and tabular alumina of Examples l and II, are made in amanner similar to that described in Examples l and II. Lower freezingtemperatures can be used to cut down the time necessary to freeze eachstuccoed layer.

Freezing temperatures which are very much in excess of ap proximately F.should not be used, as the time for freezing will be lengthened to anexcessive degree and the resultant frozen layers will not be as strong,as when the layers are frozen in a shorter period of time. Temperaturesin excess of +5 F. are impractical for use.

CONCLUSION discovered whereby stuccoed layers can be frozen on to ofeach other to form a composite ceramic shell mold in a s ort period oftime.

The invention is hereby claimed as follows:

1. The process of producing a ceramic shell mold which comprises forminga ceramic shell by the steps of:

a. Coating an expendable pattern with a ceramic slip comprising anaqueous colloidal silica sol vehicle having a granular refractorymaterial suspended therein,

b. Stuccoing the coated pattern with a dry portion of said granularrefractory material to form a stuccoed layer,

c. Freezing the stuccoed layer at a temperature below +5 d. Forming aplurality of superimposed similar layers on the pattern to form a frozenceramic shell, and

e. Removing the pattern from the ceramic shell by heating the pattern toa temperature of at least 700 F. for a period of time sufficient toremove the pattern from the ceramic shell.

2. The process of claim I in which the granular refractory material isat least one member of the group consisting of fused silica, crystallinesilica, zircon, mullite, alumina, calcined alumino-silicate, magnesia,chromia and magnesium silicate.

3. The process of claim 2 in which the refractory material is fusedsilica.

4. The process of producing a ceramic shell mold which comprises forminga ceramic shell by the steps of:

a. Coating an expendable pattern with a ceramic slip comprising anaqueous alumina coated colloidal silica sol vehicle having a granularrefractory material suspended therein,

b. Stuccoing the coated pattern with a dry portion of said granularrefractory material to form a stuccoed layer,

c. Freezing the stuccoed layer at a temperature below +5 d. Forming aplurality of superimposed similar layers on the pattern to form a frozenceramic shell, and

. Removing the pattern from the ceramic shell by heating the pattern toa temperature of at least 700 F. for a period of time sufficient toremove the pattern from the ceramic shell.

5. The process of claim 4 in which the granular refractory material isat least one member of the group consisting of fused silica, crystallinesilica, zircon, mullite, alumina, cal cined alumino-silicate, magnesia,chromia and magnesium sil icate.

6. The process of claim 5 in which the refractory material is alumina.

32 3; UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,583,468 Dated June 8, 1971 Inventor(s) Alan K. Cutler It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3 Table I, underneath"pH", "A1 0 /5110; Ratio-- should appear infirst column of table Signed and sealed this 2nd day of November 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Patents

2. The process of claim 1 in which the granular refractory material is at least one member of the group consisting of fused silica, crystalline silica, zircon, mullite, alumina, calcined alumino-silicate, magnesia, chromia and magnesium silicate.
 3. The process of claim 2 in which the refractory material is fused silica.
 4. The process of producing a ceramic shell mold which comprises forming a ceramic shell by the steps of: a. Coating an expendable pattern with a ceramic slip comprising an aqueous alumina coated colloidal silica sol vehicle having a granular refractory material suspended therein, b. Stuccoing the coated pattern with a dry portion of said granular refractory material to form a stuccoed layer, c. Freezing the stuccoed layer at a temperature below +5* F., d. Forming a plurality of superimposed similar layers on the pattern to form a frozen ceramic shell, and e. Removing the pattern from the ceramic shell by heating the pattern to a temperature of at least 700* F. for a period of time sufficient to remove the pattern from the ceramic shell.
 5. The process of claim 4 in which the granular refractory material is at least one member of the group consisting of fused silica, crystalline silica, zircon, mullite, alumina, calcined alumino-silicate, magnesia, chromia and magnesium silicate.
 6. The process of claim 5 in which the refractory material is alumina. 